Turbocharged engines, in particular turbocharged two-stroke engines, are commonly used in marine vessels for propulsion. Turbocharged two-stroke engines for marine vessels are furthermore usually configured as diesel engines, also known as compression-ignition engines. In a two-stroke diesel engine fuel is injected after air is compressed in the combustion chamber, causing the fuel to self-ignite. However, it is also possible that a fuel air mixture is injected into the combustion chamber and compressed until the fuel air mixture self-ignites.
After the expansion of the ignited fuel or air fuel mixture a scavenge gas, in particular scavenge air, is injected into the combustion chamber. The scavenge gas can be injected into the combustion chamber via the air inlet port or the air fuel mixture inlet port of the combustion chamber, respectively, or via a separate scavenge port. For scavenging, two-stroke diesel engines require a mechanically driven blower or a turbocharger to charge the cylinder. In a turbocharger a gas, in particular air, is compressed. Due to the compression the temperature of the gas is increased. However, an increased temperature of scavenging gas or charge gas, respectively, decreases the efficiency of the turbocharged two-stroke engines. Therefore, the compressed gas is subsequently cooled down in a charge gas cooler, in particular a charge air cooler. Charge gas coolers are also known as intercoolers. The compressed, pressurized and cooled down charge gas is then injected into the combustion chamber of the two-stroke diesel engine to scavenge the combustion chamber and to provide the gas, in particular air, to charge the combustion chamber for the next compression and ignition cycle.
However, as the temperature of the charge gas decreases in the charge gas cooler, the relative water content or humidity of the charge gas increases and may increase to a level where the humidity reaches saturation. Water may then condense out of the charge gas and pool in a scavenge gas duct or charge gas duct, respectively. Depending on the efficiency of the water catcher of the engine and the pressure of the scavenge gas or charge gas, the water condensate may be dragged with the flow of scavenge gas or charge gas into the combustion chamber and increase the risk of corrosion of components of the combustion chamber or even disturb the oil film between piston ring and liner and may lead to abrasive wear or scuffing.
For a diesel two-stroke engine in slow steaming mode it would be optimal to keep the temperature of the charge gas or scavenge gas as low as possible so that a maximal amount of water condenses out of the charge gas because in slow steaming mode the water catcher can operate more efficiently. However, in full load mode the mass flow rate of charge gas is very high and, thus, the risk that water condensate is dragged into the combustion chamber increases. With the increased risk of water entering the combustion chamber the risk of scuffing and corrosion of components of the combustion chamber increases as well. Therefore, in prior art, a higher charge gas temperature is chosen to reduce the mass flow rate of the charge gas and to reduce the risk of water condensate being dragged into the combustion chamber.
An apparatus for controlling the charge air temperature of an internal combustion machine is disclosed in EP 0 508 068 A1. The dew temperature of the charge air is determined from measured values of the humidity and the temperature of the charge air and by raising the charge air temperature the condensation of water vapor in the charge air is prevented.
The present invention is based on the finding that contrary to the assumptions in prior art a higher charge gas temperature does not decrease the mass flow rate of water into the combustion chamber because due to the higher charge gas temperature more water gets into the combustion chamber in form of water vapor. Furthermore, the present invention takes also into account that almost the same amount of water is generated in the combustion process as is injected into the combustion chamber via the charge air or scavenge air.